The present invention relates generally to acceleration responsive devices, and more specifically to an accelerometer employing a permanent magnet/moving coil rebalance system and unique temperature compensation means for minimizing variations in accelerometer response due to changes in temperature.
Acceleration responsive devices perform essential sensing functions in a wide variety of systems. As performance requirements and available technology have advanced, the demand has increased for sensors characterized by much improved sensitivity, stability, accuracy, linearity of response, reliability and ruggedness, in addition to fast reaction time and low cost. Implicit in the stability, sensitivity and response linearity requirements is a requirement that precision be maintained over a wide temperature range. The present state of the art is such that it has been difficult to achieve improvements in all of the foregoing characteristics simultaneously, or, in some instances, even to achieve improvement in one characteristic without adversely affecting another. Nevertheless, requirements exist, particularly in aircraft navigation and missile guidance systems, for a single acceleration sensor with superior performance in all of the noted areas.
One of the functions which must be provided for in a nonintegrating accelerometer is that of returning the acceleration sensing mass to a predetermined rest position after it has been displaced by an acceleration. A variety of system variations for performing this rebalance function have been devised and are presently employed. One such system variation described as a permanent magnet/moving coil rebalance system has shown considerable promise in simultaneously meeting the performance and low cost requirements. However, an inherent characteristic of permanent magnet/moving coil systems is that any variation in magnetic flux density in the region surrounding the moving coil results in a variation in the magnitude of response resulting from a given input acceleration, or variation in scale factor. One source of variation in magnetic flux density stems from the temperature dependent permeabilities of known magnetic materials. Although the most stable magnetic materials have comparatively small temperature dependencies, no such material has been found which can meet the requirements for the most demanding accelerometer applications.
One known technique for improving accelerometer performance under varying temperature conditions is to provide a stable temperature environment for the accelerometer by means of a heater or other temperature control device. However, suitable forms of temperature control are complex, costly and result in slow accelerometer reaction time. Thus, the accelerometer designs requiring temperature control are not suitable for the most demanding aircraft navigation and missile guidance systems, and further do not meet the low cost objective dictated for many applications.
The applicant has avoided the above-described problems by providing a unique system for augmenting the magnetic field(s) produced in permanent magnet/moving coil accelerometer rebalance apparatus. The compensation system is simple, reliable and low in cost, and does not detract from the fast reaction time potential of permanent magnet/moving coil accelerometers. Accelerometers employing the applicant's temperature compensation system have been found capable of performance and reliability heretofore unavailable from simple low cost apparatus.